What is the spirit of Elijah? Does it mean the fervor with which Elijah went about his duty? Or literally his..."essence" coming forth to be among people again? I know the Messiah said John the Baptist was "Elijah", yet, doesn't Revelations speak of it as well?
I'm guessing you are referring to Luke 1:17 "He [the baby who would become John the Baptist] will go out ahead of Adonai in the spirit and power of Eliyahu to turn the hearts of fathers to their children and the disobedient to the wisdom of the righteous, to make ready for Adonai a people prepared.
Eliyahu was the prophet sent by YHWH to the Northern Kingdom of Israel at a time when Israel had largely forsaken YHWH and turned to the worship of idols, under the influence of Israel's king at that time, Ahab (whom the Bible records [1 Kings 16:30] as committing more evil than any of the kings of Israel before him), in concert with his pagan wife Jezebel. Idol worship during his reign was so prevalent that there were only 7,000 people in the entire kingdom of Israel who had not bowed down in worship to Ba'al (1 Kings 19:18).
Eliyahu's prophetic mission was to bring Israel back to the worship of YHWH. He repeatedly warned people to repent of their sin and return to YHWH, and warned them of the judgment of YHWH that awaited them if they did not.
So the "spirit of Eliyahu" would be one who has the same mission and zeal as Eliyahu.
Eliyahu is not mentioned in Revelation though many read Revelation 11 and conclude that the "two witnesses" represent Eliyahu and Enoch or Eliyahu and Moses.